1. Field of the Invention
The present invention relates to an optical measuring device that can measure the total luminous flux of a light source.
2. Description of the Related Art
Among various optical measuring devices, a total luminous flux measuring device can measure the total luminous flux of a surface emitting light source such as a display with respect to a total luminous flux standard lamp, which is a cylindrical light source. A typical conventional total luminous flux measuring device includes a perfect integrating sphere, of which the inner surface is coated with a perfectly diffuse reflective material such as barium sulfate. A sample lamp, which is the object of measurement, is arranged at the center of the integrating sphere and its luminous flux is measured through an observation window, which is located on the surface of the integrating sphere. A baffle is arranged between the observation window and the sample lamp such that the light emitted from the sample lamp does not enter the observation window directly. By measuring and comparing the luminous flux of the sample lamp to the known total luminous flux of a total luminous flux standard lamp using such a sphere photometer, the total luminous flux of the sample lamp can be obtained.
In such a sphere photometer, the sample lamp needs to be lit at the center of the integrating sphere, thus requiring a supporting member to fix the sample lamp at the center of the integrating sphere. However, since such a supporting member and the lamp itself absorb light to cause a measurement error, the supporting member is often coated with the same material as that applied onto the inner surface of the integrating sphere.
According to a proposed method, a light source for measuring self-absorption is lit on the inner surface of the integrating sphere, and the self-absorption of the supporting member and the sample lamp is calculated as the ratio of the output of the measuring device with the supporting member and the sample lamp to that of the measuring device without them. Actually, however, the lamp supporting member often serves as a wire duct to light the lamp as well and is often fixed on the integrating sphere. That is why the self-absorption ratio of the only the sample lamp is usually calculated (see JIS C7607-1991, Method for Measuring the Total Luminous Flux of Light Measuring Standard Electrical Discharge Lamp, Appendix: How to Calculate Correction Coefficient, 2. How to Define Self Absorption Correction Coefficient k2 according to Various Lamp Shapes or Sizes).
On the other hand, the spatial and spectral distributions of the light emitted from the sample lamp are different from those of the total luminous flux standard light source, and therefore, the self-absorption of the lamp supporting member and the sample lamp has a non-negligible value.
To overcome such a problem, a novel total luminous flux measuring device, including a hemisphere and a plane mirror, was proposed in Japanese Patent Application Laid-Open Publication No. 6-167388 (see FIG. 1).
The device disclosed in Japanese Patent Application Laid-Open Publication No. 6-167388 (see FIG. 1) is fabricated by providing an integrating hemisphere 2, of which the inner surface is coated with a light diffuse reflective material 1 such as barium sulfate, and closing the opening of this integrating hemisphere 2 with a plane mirror 3 as shown in FIG. 8. A hole 5 has been cut through the plane mirror 3 so as to be located at the center of curvature of the integrating hemisphere and to receive a light source 4 under measurement. By lighting the light source 4 under measurement inside the integrating hemisphere 2, a virtual image of the inner wall of the integrating hemisphere 2 and the light source 4 under measurement is formed by the plane mirror 3. As a result, the light source 4 under measurement and a virtual image thereof are both lit inside an integrating sphere having the same radius as the integrating hemisphere. In this manner, the total luminous flux of the two light sources, consisting of the light source 4 under measurement and a virtual image thereof, is measured by a photodetector 6.
In this device, the lamp supporting member (lighting jig) 8 is arranged outside of the integrating space, and therefore, the total luminous flux measured is not affected by the self-absorption produced by the lamp supporting member 8. Consequently, high measuring accuracy is realized without performing any complicated process to correct the self-absorption of the lamp supporting member 8, for example. Also, since the integrating space is only a half of a full integrating sphere, the photodetector 6 can have doubled illuminance at its light receiving window. As a result, the SNR can be increased in measuring the total luminous flux.
According to the arrangement shown in FIG. 8, however, the baffle 7 needs to cut off not only the light coming directly from the light source 4 under measurement but also the light coming directly from the virtual image of the light source 4 under measurement. That is why the size of the baffle 7 has to be more than doubled compared to the situation where only the light source 4 under measurement is lit inside a full integrating sphere as will be described later. The baffle 7 inside the integrating sphere partially cuts off the optical path of the reflected light inside the integrating sphere. However, since the baffle itself absorbs light, the measurement error would increase just like the lamp supporting member arranged in the full integrating sphere.
Hereinafter, the principle of measurement to be done using the integrating sphere and the error caused due to the self-absorption of the baffle will be described in detail.
First, the principle of measurement to be done using the integrating sphere will be described with reference to FIG. 9, which shows, by way of a planar model, how the integrating sphere works.
Suppose a light source 4 is arranged at the center of an integrating sphere with a radius r and an infinitesimal surface element A on the wall of the integrating sphere has been illuminated at a luminous intensity I0(α) by the light source 4 in the direction defined by an angle α. In that case, the illuminance Ea of the infinitesimal surface element A on the integrating sphere wall is represented by the following Equation (1):Ea=I0(α)/r2  (1)
On the inner wall of the integrating sphere, perfectly diffuse reflection is produced at a reflectance ρ. Supposing the infinitesimal surface element A on the inner wall has an area dS, the luminous flux φa of the light reflected from the infinitesimal surface element A is given by the following Equation (2):φa=ρ·Ea·dS  (2)
Suppose there is an infinitesimal surface element B on the inner wall of the integrating sphere, which defines an angle θ with respect to a normal to the infinitesimal surface element A. Since the infinitesimal surface element A is a perfectly diffuse reflective surface, the luminous intensity Ia(θ) in the direction from the infinitesimal surface element A toward the infinitesimal surface element B is calculated by the following Equation (3):Ia(θ)=φa·cos θ/π  (3)
Since the surface B is located on the inner wall of the integrating sphere, the light directed toward the surface B with the luminous intensity Ia(θ) has an angle of incidence θ and the distance from the infinitesimal surface element A to the infinitesimal surface element B is 2r·cos θ. Therefore, at the surface B, the light with the luminous intensity Ia(θ) has the illuminance Eab given by the following Equation (4):
                                                                                                   E                  ab                                =                                ⁢                                                                                                    I                        a                                            ⁡                                              (                        θ                        )                                                              ·                    cos                                    ⁢                                                                          ⁢                                      θ                    /                                                                  (                                                  2                          ⁢                                                      r                            ·                            cos                                                    ⁢                                                                                                          ⁢                          θ                                                )                                            2                                                                                                                                              =                                ⁢                                                                            ϕ                      a                                        /                                          (                                              4                        ⁢                                                  π                          ·                                                      r                            2                                                                                              )                                                        =                                      ρ                    ·                                                                  I                        0                                            ⁡                                              (                        α                        )                                                              ·                                          dS                      /                                              (                                                  4                          ⁢                                                      π                            ·                                                          r                              4                                                                                                      )                                                                                                                                                    (        4        )            
As can be seen easily from Equation (4), the light reflected from the infinitesimal surface element A illuminates any portion of the inner wall of the integrating sphere with light with a uniform illuminance irrespective of the angle θ of the light that has been reflected from the infinitesimal surface element A. Since the integrating sphere has an internal surface area of 4π·r2, the very small area dS can be calculated by the following Equation (5) using a very small solid angle dΩ:dS=(4π·r2/4π)·dΩ=r2·dΩ  (5)
Thus, Equation (4) can be modified into the following Equation (6):Eab=ρ·I0(α)·dΩ/(4π·r2)  (6)
The total luminous flux φ of the light source 4 is obtained by integrating I0(α)·dΩ of Equation (6) over the entire space. Therefore, the first-order reflected light of the bundle of rays that has been emitted from the light source 4 and then reflected from the entire inner wall of the integrating sphere has an illuminance Eb1 on the surface B as represented by the following Equation (7):Eb1=ρ·φ/(4π·r2)  (7)
The first-order reflected light of the bundle of rays that has been emitted from the light source 4 and then reflected from the inner wall of the integrating sphere to have the illuminance Eb1 on the surface B is further reflected from the surface B to produce second-order reflection at a reflectance ρ. Supposing the infinitesimal surface element B has an area dS, the luminous flux φb,2 of the light reflected from the surface B is given by the following Equation (8):φb,2=ρ·Eb1·dS=ρ·Eb1·r2·dΩ  (8)
Since the surface B is a perfectly diffuse reflective surface, the luminous intensity Ib(θ) of the light reflected from the surface B at the angle θ is calculated by the following Equation (9):Ib(θ)=φb,2·cos θ/π  (9)
Supposing there is a point C that defines the angle θ with respect to the surface B, the illuminance Ebc of the light with the luminous intensity Ib(θ) on the surface B is given by the following Equation (10):Ebc=Ib(θ)·cos θ/(2r·cos θ)2=φb,2/(4·r2)=ρ·{ρ·φ·dΩ}/(4π·r2)  (10)
Consequently, the second-order reflected light from the surface B will illuminate any portion of the inner wall of the integrating sphere at a constant illuminance irrespective of the angle θ at which the light has been reflected from the surface B. That is to say, the second-order reflected light from the entire inner wall of the integrating sphere will have an illuminance on the surface B, which is calculated by integrating dΩ of Equation (10) over the entire space and represented by the following Equation (11):Eb2=ρ2·φ/(4π·r2)  (11)
Supposing the light emitted from the light source 4 directly toward the surface B has a luminous intensity I0(β) and considering high-order reflected light that follows the first-order one, the illuminance Eb of the light on the surface B is given by the following Equation (12):
                                                                                                                       E                    b                                    =                                    ⁢                                                                                    I                        0                                            ⁢                                                                        {                          β                          }                                                /                                                  r                          2                                                                                      +                                          ρ                      ·                                              ϕ                        /                                                  (                                                      4                            ⁢                                                                                                                  ⁢                                                          π                              ·                                                              r                                2                                                                                                              )                                                                                      +                                                                  ρ                        2                                            ·                                              ϕ                        /                                                  (                                                      4                            ⁢                                                                                                                  ⁢                                                          π                              ·                                                              r                                2                                                                                                              )                                                                                      +                                                                  ρ                        3                                            ·                                              ϕ                        /                                                  {                                                      4                            ⁢                                                          π                              ·                                                              r                                2                                                                                                              }                                                                                                                                                                                          =                                    ⁢                                                                                                              I                          0                                                ⁡                                                  (                          β                          )                                                                    /                                              r                        2                                                              +                                          ρ                      ·                                              ϕ                        /                                                  {                                                                                                                    (                                                                  1                                  -                                  ρ                                                                )                                                            ·                              4                                                        ⁢                                                          π                              ·                                                              r                                2                                                                                                              }                                                                                                                                                  ⁢          …                                    (        12        )            
If the light source 4 produces a spherical spatial distribution of light, the illuminance Eb,0 of the light that has come directly from the light source 4 to have the illuminance Eb on the surface B is calculated by the following Equation (13):Eb,0=I0(β)/r2=φ/(4π·r2)  (13)
On the other hand, the illuminance Eb,r of the light that has been reflected from the inner wall of the integrating sphere to have the illuminance Eb on the surface B is calculated by the following Equation (14):Eb,r=ρ·φ/{(1−ρ)·4π·r2}  (14)
The ratio of the illuminance Eb,0 of the light that has come directly from the light source 4 to the illuminance Eb,r of the light that has been reflected from the inner wall of the integrating sphere is given by the following Equation (15):
                                                                                                                       E                                          b                      ,                      0                                                        ⁢                                      :                                    ⁢                                      E                                          b                      ,                      r                                                                      =                                ⁢                                                      ϕ                    /                                          (                                              4                        ⁢                                                  π                          ·                                                      r                            2                                                                                              )                                                        :                                      ρ                    ·                                          ϕ                      /                                              {                                                                                                            (                                                              1                                -                                ρ                                                            )                                                        ·                            4                                                    ⁢                                                      π                            ·                                                          r                              2                                                                                                      }                                                                                                                                                                    =                                ⁢                                  1                  :                                      ρ                    /                                          (                                              1                        -                        ρ                                            )                                                                                                                              (        15        )            
This ratio is determined by the reflectance ρ of the light diffuse reflective surface 1. For example, if the reflectance is approximately 95%, then the light that has come directly from the light source 4 will have an illuminance Eb,0 of approximately 5%. This value is obtained when the light source 4 produces a spherical spatial distribution of light and is subject to change significantly with the spatial distribution of the light to be produced by the light source 4 under measurement.
For that reason, the conventional integrating sphere has an observation window at the wall surface B of the integrating sphere. And if a photodetector 6 with corrected luminous efficacy is arranged at the observation window, the light that has come directly with the luminous intensity I0(β) from the light source 4 is cut off by the baffle 7. As a result, the photodetector 6 can measure an illuminance that is proportional to the total luminous flux of the light source 4.
Next, an error caused due to the self-absorption of the baffle will be described with reference to FIGS. 10(a) and 10(b).
Suppose the baffle 7 is arranged between the light source 4 and the photodetector 6 as shown in FIGS. 10(a) and 10(b). In that case, when looked through the light receiving window of the photodetector 6, the illumination will be absent from the p-q range on the inner wall of the integrating sphere as shown in FIG. 10(a). On the other hand, when looked from the light source 4, the p′-q′ range on the inner wall of the integrating sphere cannot be illuminated directly as shown in FIG. 10(b). As a result, measurement errors are likely to be caused in both of these situations. This error will increase as the baffle 7 gets bigger. However, the larger the light source 4 under measurement, the bigger the baffle 7 has to be.
According to the arrangement shown in FIG. 8, the baffle 7 for cutting off the light that has come directly from the light source 4 under measurement is put on the plane mirror 3. In that case, the baffle 7 has to cut off the light that has come directly from the two light sources, namely, the light source 4 under measurement and its virtual image, as shown in FIG. 11. That is why in the arrangement shown in FIG. 8, the size of the baffle 7 has to be more than doubled compared to the situation where only the light source 4 under measurement is lit in the full integrating sphere, thus increasing the magnitude of measurement errors.
In order to overcome the problems described above, the present invention provides an optical measuring device that can measure the total luminous flux highly accurately with such measurement errors due to the presence of the baffle reduced significantly by eliminating the baffle.